Several focussing detectors have been proposed in which, for example, a non-linear characteristic of cadmium sulfide (CdS) exposed to light is utilized where a plurality of minor photoconductive elements are arranged to extract a contrast of images on the surfaces of these elements from an object to be photographed. In using any one of these proposed photoelectric focussing detectors for a single lens reflex camera, the photoelectric focussing detector has usually been located at a position optically equivalent to the film defined plane with respect to the reflective mirror disposed in the path of light coming through an objective lens from an object to be photographed. With such arrangement for a photoelectric focussing detector, the detector is responsive only to the movement of the objective lens for generating a focussing signal so that the mechanical structure of the lens system suffer from several disadvantages. Namely, when the objective lens is out of focus, the image of an object to be photographed is also out of focus, and no signal indicating such out of focus is generated by the focussing detector. Additionally, a slight movement of the objective lens from this state causes no variation in the output of the focussing detector. Accordingly, it is impossible to determine the direction in which the objective lens should be moved in order to obtain proper focussing.
Typically, photoconductive elements arranged for use in a focussing detector provide a response characteristic as a function of the position of the objective lens that defines non-responsive zones about either side of a responsive zone within which the point of proper focussing exists. It is possible within the respective zone to move the objective lens to the focussing position based upon determination of the direction in which the output of the focussing detector varies as the objective lens is moved, whereas, it is impossible within the non-responsive zones to determine the direction in which the objective lens may be moved in order to bring the lens to the focussing position since no variation occurs in the output of the focussing detector. With the photographic camera of the automatic focussing type utilizing the photoelectric focussing detector as mentioned above, there occurs a probability of 50% at which the objective lens is moved in the direction opposite to the proper focussing position when the objective lens lies initially within any one of the non-responsive zones.
An object of the present invention is to realize a photographic camera of automatic focussing type utilizing a photoelectric focussing detector in which a simplified mechanical structure is utilized and the foregoing described problems are minimized.
In accordance with the present invention, the objective lens lying initially within any one of the non-responsive zones is moved at a high velocity to move the objective lens out of the non-responsive zone within a relatively short period of time even if the objective lens has been moved in the direction opposite to the focussing position. A switch associated with focussing an object at the infinite distance or a switch associated with focussing an object at the minimum distance reverses an objective lens moving focussing motor upon arrival of the objective lens at the position for photographing an object at infinity or the minimum distance so as to reverse the movement of the objective lens into the proper direction. When the objective lens being moved at such high velocity enters into the responsive zone, the speed of moving the objective lens is reduced in order to provide focussing position and to prevent the objective lens from moving beyond the focussing position due to its inertia. Thus, hunting or oscillation of the objective lens relative to the focussing position is prevented and the objective lens stops at the focussing position.